1. Technical Field
Aspects of the present invention relate to a power factor correction type switching power supply unit that supplies a direct current output voltage to a load based on a full-wave rectified alternating current input voltage, and in particular relate to a power factor correction type switching power supply unit that enables an overcurrent limit value to be changed to two differing magnitudes in accordance with the size of the alternating current input voltage.
2. Related Art
In recent years, a switching power supply unit that has an alternating current voltage as an input has been widely utilized in electronic instruments. This kind of switching power supply unit being one which, by causing a switching operation of a switching element linking an input and an output, converts a full-wave rectified alternating current input voltage into a direct current output voltage of a desired size, and supplies it to a load as, for example, the one described in JP-A-7-7953 (refer to paragraphs [0012] to [0018], FIG. 1, and the like of that reference), to be described hereafter, is known.
FIG. 6 is a circuit diagram showing one example of a heretofore known power factor correction type switching power supply unit. Herein, a power factor correction (PFC) type switching power supply circuit that operates in continuous conduction mode is shown, and this is applied to an active filter type power supply unit.
The heretofore known power factor correction type switching power supply unit shown in FIG. 6 has a full-wave rectifier 4 that full-wave rectifies a commercial power supply 2, and its output is connected to one end of an inductor L1. The connection point of the other end of the inductor L1 and a diode D1 is connected to the drain terminal of, for example, an N-channel type MOS transistor (a metal oxide semiconductor field-effect transistor) functioning as a switching element 6. The other end of the inductor L1 is connected to a load 8 via a rectifying and smoothing circuit formed of the diode D1 and a capacitor C1, and a direct current voltage Vout is output to the load 8.
As well as the source terminal of the MOS transistor, which is the switching element 6, being connected to the ground (GND), the gate terminal is connected to an output terminal DO of a power factor correction control circuit 10. One end of a series resistor circuit formed of resistors R1 and R2 is connected to the connection point of the full-wave rectifier 4 and inductor L1, and the other end is grounded. A multiplier input terminal VDET of the power factor correction control circuit 10 is a terminal into which a detected value of an alternating current output voltage is input from the full-wave rectifier 4, and the connection point of the resistors R1 and R2 is connected to the multiplier input terminal VDET. Also, the full-wave rectifier 4 is grounded via a resistor R3, and the connection point of the full-wave rectifier 4 and resistor R3 is connected to an inductor current signal input terminal IS of the power factor correction control circuit 10. Furthermore, a series circuit of resistors R4 and R5 is connected in parallel with the load 8, and a direct current output voltage Vout the same as that of the load 8 is applied thereto. A feedback voltage input terminal FB of the power factor correction control circuit 10 being a terminal into which a detected value of the direct current output voltage Vout is input, herein, the connection point of the resistors R4 and R5 is connected to the feedback voltage input terminal FB, and a voltage signal wherein the direct current output voltage Vout is voltage divided is returned here.
Next, a simple description will be given of an operation of the heretofore described heretofore known power factor correction type switching power supply unit of FIG. 6.
The heretofore known power factor correction type switching power supply unit of FIG. 6 employs a control method called an average current control method, average current mode control, or the like, and the power factor correction control circuit 10 is one that sinusoidally controls a current flowing to the alternating current commercial power supply 2 side in the same phase as that of the alternating current input voltage, while stabilizing the direct current output voltage Vout. The feedback voltage input terminal FB of the power factor correction control circuit 10 is connected to an input terminal of a voltage error amplifier 14, formed of a transconductance amplifier, together with a reference voltage source 12 which sets a voltage command value for the direct current output voltage Vout. The voltage error amplifier 14 generates a voltage error amplification signal wherein the difference between the detected value (a divided voltage value in this case) of the direct current output voltage Vout and the voltage command value (for example, 2.5V) of the reference voltage source 12 is amplified. A capacitor C2 and a series circuit of a resistor R6 and capacitor C3 are connected between the output terminal of the voltage error amplifier 14 and the GND. The voltage error amplification signal is supplied to a first input terminal of a multiplier 16.
A second input terminal of the multiplier 16 is connected to the multiplier input terminal VDET of the power factor correction control circuit 10, and the detected value (a divided voltage value in this case) of the alternating current output voltage of the full-wave rectifier 4 is input from here. The multiplier 16 multiplies the voltage error amplification signal supplied to the first input terminal and the detected value Vdet of the alternating current output voltage of the full-wave rectifier 4 supplied to the second input terminal, and makes this the value of a current command to a current error amplifier 18.
An inductor current signal, which is a voltage signal wherein an inductor current IL from the inductor current signal input terminal IS is voltage converted in the current detecting resistor R3, and an output signal Vmul of the multiplier 16, which is the current command value, are input into the current error amplifier 18. Also, an overcurrent protection (OCP) circuit 24 is connected to the inductor current signal input terminal IS. A sawtooth wave or triangular wave carrier signal of a constant frequency that determines a switching cycle is generated in an oscillator circuit (OSC) 20, and input into a PWM comparator 22. In the PWM comparator 22 into which the carrier signal and the current error amplification signal are input, the magnitudes of the signals are compared, a pulse width modulation (PWM) control signal is generated, and this is applied to the gate terminal of the switching element 6 via an AND circuit 26 and driver circuit 28.
Herein, the overcurrent protection circuit 24, based on the inductor current signal, limits the peak value of the inductor current IL in every switching cycle of the switching element 6. Herein, when an inductor current exceeding a predetermined threshold value flows, an L (Low) level overcurrent limit signal is input into the AND circuit 26, and the output of the AND circuit 26 compulsorily becomes L. A switching signal is output to the output terminal DO of the power factor correction control circuit 10 from the AND circuit 26 via the driver circuit 28. By controlling the on-off timing of the switching element 6 in this way, it is possible to control the value of a current flowing to the capacitor C1 via the diode D1. While a feedback constant setting circuit is connected between the input and output terminals of the current error amplifier 18, for clarity of description, a depiction of the feedback constant setting circuit is omitted from FIG. 6.
When this kind of power factor correction type switching power supply unit has the alternating current commercial power supply 2 as an input, it is supposed that the range of its alternating current input voltage is wide at 80 Vac to 265 Vac. Then, even with a switching power supply unit in which it is supposed that the alternating current input voltage changes, it is possible to realize a constant power control that places a limit on the direct current output power, but in actuality a circuit scale becomes large. For this reason, the constant power control is not normally applied to a switching power supply unit configured of a semiconductor integrated circuit with a small number of pins. That is, as the power factor correction control circuit 10 configured of a semiconductor integrated circuit with a small number of pins does not have a constant power control function, there is a problem in that, even when the overcurrent protection circuit operates at the same threshold value as the inductor current, the direct current output power changes greatly depending on the value of the alternating current input voltage.
FIG. 7 is a diagram showing a relationship between a direct current output power limited by an overcurrent protection operation and an alternating current input voltage. An alternating current input voltage (Vac) is shown on the horizontal axis, and a limited power (W) on the vertical axis.
That is, when the alternating current input voltage is high, the inductor current increases, and it becomes difficult to keep the direct current output power within an appropriate range, even when carrying out an overcurrent limitation. Therein, in a rectifying device including a function limiting the overcurrent of an AC input current, the rectifying device using a step-up converter type active filter, an overcurrent limiting circuit that detects whether the AC input voltage system is a 100V system or a 200V system and, in the case of the 200V system, switches the limit value of the overcurrent to one half of that of the 100V system, is disclosed in, for example, JP-A-7-7953 (refer to paragraphs [0012] to [0018], FIG. 1, and the like of that reference).
Also, in JP-A-2003-219635 (refer to paragraphs [0009] to [0023], FIGS. 3, 4, and the like of that reference) too, a technology is disclosed whereby a comparison reference voltage Vth for the inductor current is changed between a 100V system and a 200V system in order to avoid overcurrent limiter levels differing drastically between the 100V system and 200V system.
In neither of the technologies disclosed in the heretofore described JP-A-7-7953 and JP-A-2003-219635 is there any mention of a switch timing when switching between the two limit values or two comparison reference values. That is, there is no limit on the switch timing of the limit values or comparison reference values.
However, when providing the power factor correction switching power supply unit with an overcurrent limiting function having two threshold values, when the alternating current input voltage (the absolute value thereof) is high in order to realize a power factor correction, the input current also increases, meaning that the following kinds of problem occur.
That is, when setting overcurrent detection reference values and observing the alternating current input voltage on the premise that the alternating current input voltage is low, a case can be supposed wherein it is determined that the input voltage is high, and the reference value is switched. Therein, a switch is made from the higher reference value of the two threshold values to the lower reference value.
However, determining that the alternating current input voltage is high means that the detected input current is also increasing. Then, suddenly lowering the reference value of the overcurrent limit in that kind of situation is a movement compulsorily shutting off the current flowing in the inductor of the switching power supply unit. When the switching power supply unit is operating in a continuous current mode, as the inductor current is inevitably also at a high value at the point at which it is determined that the alternating current input voltage is high, there is a problem in that when attempting to shut off the current, an oscillation of the inductor current, and an accompanying squeaking, occur.
Also, when calculating the average value of the alternating current voltage using a low pass filter, or the like, and switching the overcurrent detection reference value in accordance therewith, it is not possible to predict the switch timing. Consequently, conditions of the inductor current oscillation and squeaking become complicated at a time of an overcurrent detection, and there is a problem in that there is a serious impediment to the stable operation of the power factor correction type switching power supply unit.